1. Field of the Invention
The present invention relates to a hologram recording/reproducing device, a hologram recording/reproducing method, and a holographic recording medium.
2. Description of the Related Art
In recent years, a holographic memory has been popular as a high-capacity recording medium. The holographic memory is a memory using a holographic recording medium. For example, when a tilt amount of a holographic recording medium during recording differs from a tilt amount during reproducing by 0.05°, a brightness of a diffraction image may become uneven, and a diffraction efficiency may be attenuated to 67%. To prevent an error rate from being further decreased, a tolerance of tilting has to be further decreased to, for example, about 0.01°. As compared with a tolerance of tilting for a typical optical disk, for example, a DVD disk, the tolerance value of the holographic memory is a several-tenth part the tolerance value of the DVD disk, the tolerance value of the holographic memory being markedly small. Here, the tilt amount during recording differing from the tilt amount during reproducing represents that the tilt amounts vary during recording and reproducing. Even when the tilt amount during recording is about 0.n° (n being an integer), reproduction can be carried out as long as a difference between the tilt amount during recording and a tilt amount during reproducing is within, for example, 0.01°. That is, for a holographic memory, the relative difference between the tilt amount during recording and the tilt amount during reproducing is a key point.
Owing to this, in a hologram recording/reproducing device (holographic memory system) using such a holographic memory, a tilt amount is detected and tilting is corrected to decrease the tilt amount. For example, Japanese Unexamined Patent Application Publication No. 2005-32309 suggests a technique in which optical beams of recording light and reproducing light, to be incident on an objective lens, are moved parallel. Also, Japanese Unexamined Patent Application Publication No. 2007-141426 suggests a technique, in which, when using two-beam interference method, reproducing light is moved in radial and tangential directions, and reproduction is carried out in a state where SNR is in an optimum condition. Two-beam interference method and coaxial interference method are basic techniques for recording and reproducing a hologram.